The present invention relates to a four cycle engine provided with a diaphragm type carburetor, and more particularly to an engine for a portable working machine, in which pulsating pressure for driving a fuel pump may be picked up.
In a conventional engine for a portable working machine such as a trimmer, a posture of the engine is not kept constant during the operation, and it often operates under the condition that a crank shaft of the engine is slanted at angles 40.degree. to 50.degree. in the back and forth and right and left directions relative to a horizontal position. Accordingly, due to the positional relationship of the fuel supply device such as a carburetor, two-cycle engines are more frequently used instead of four-cycle engines. As shown in FIG. 9, the conventional two-cycle engine includes a crank shaft 1, a piston 2, a connecting rod 3, a cylinder 4, an intake port 5 formed in the cylinder 4, a carburetor 6, a heat insulator 7 installed between the cylinder 4 and the carburetor 6, and pulsating pressure pickup ports 8 and 8a which are in communication with a diaphragm type fuel pump chamber (not shown) of the carburetor and a crank chamber space S1 and the like. As well known in the two-cycle engine, the intake port is interrupted from the crank chamber space S1 by the downward stroke of the piston 2, whereas the intake port is in communication with the crank chamber space S1 by the upward stroke of the piston 2. In this case, the pressure within the crank chamber is changed, so that the pulsating pressure generated thereby may be utilized as a drive source for operating the fuel pump within the carburetor. The smaller the volume of the space S1, the large the pressure change of the space S1 caused by the reciprocating motion of the piston will become.
Also, in a four-cycle engine, as shown in FIG. 10, a cam shaft 9 and the like as well as a crank shaft 1 are disposed in a crank chamber space S2, and a volume of the space S2 is larger than the space S1 of the two-cycle engine even if a volume of lubricant 26 is subtracted from the volume of the space S2. Accordingly, a pulsating pressure enough to drive the fuel pump of the diaphragm type carburetor will not be generated.